The cats are probably bored. It's good that they have cat trees and boxes, but they need more enrichment. There are lots of options, both fancy and simple.

Just saying 'no' to any animal means nothing. They could go the spray bottle route, or try putting things that smell bad to cats (mine runs from citrus and lavender smells) on the tables. They need to stop worrying about being the 'bad guy' and take care of their pets.

Do NOT trade cats with them. Your cat will only learn bad habits in their hands.

The cats being bored was my first thought. I also agree that they are going to have to be the "bad guy" and actually do something to discipline the cats to keep them out of things that they're supposed to not be in and to keep them safe and such. I don't know why he thinks that the cats will stop liking them. I spray mine with a water bottle or something if they're somewhere they aren't supposed to be, and they definitely still like me. One of them gets in trouble all the time and he still sleeps curled up next to me and hasn't started hating me yet, and I've had him his entire life, so I'm not sure why they think that would happen.

I wouldn't trade with them anyway. My cats are all pretty attached to each other.

, it's great that you're being sensible about it.

As a cat-herder, I'm sure you've seen all the everyday things that will entertain a cat (crumpled paper, empty mailing box, etc). It shouldn't be too hard to provide enough toys to distract them.

Maybe give your friends a gift of a small oil diffuser, one without reeds? Give it with some eucalyptus or citronella oil.

Yeah, cats are pretty easy. Literally anything entertains mine. One of them was playing with a random loose thread from a pair of pants this morning. I like the idea of giving them an oil diffuser. That seems helpful. Maybe I'll also make them some cat toys.

All it takes to make mine happy is for me to let her roll on my bare feet. Cats are weird animals.

Cats are really odd. One of mine likes to lay in the sink or in clean laundry, and that's basically the thing that makes him happiest.

Contrary to popular belief, cats aren't animals that you can just leave by themselves for hours on end. It sounds like your friend and his SO don't really have the means to properly care for their animals. Getting a second cat was a bad call if the first one was acting out. Pets aren't windup playthings; they need proper attention and an enriching environment. If your friend and his SO want their cats to be happy and less destructive then they need to make some changes to their schedules so they can spend more time playing with the cats. They would also benefit from putting up climbing shelves and, yes, closing doors when applicable. If your friend can't even manage to keep a few doors closed to keep the cats out of his important stuff then he isn't ready to own a cat, period. If they can't make any accommodations for these animals then they need to give them to a person or an organization that can.

That whole shocking mat thing actually kind of concerns me? Like...you don't have to hurt your animals to make them behave. Your friend and his SO are doing something wrong. Honestly, at this point there's not much you can do for him except telling him that pets aren't right for him. After that, establish your own boundaries and tell him you don't have anymore advice for him. If you think the cats are endangered by his actions, then maybe you could also call an organization that will get them out of the house and into a safer environment.